Speech codecs using Adaptive Multi Rate (AMR) [1,2] enable provision of excellent speech quality and provide at the same time a way forward towards state-of-the-art, spectrally efficient, high capacity cellular networks. One straight-forward way to characterize the benefit of AMR speech codecs is that the robustness to interference and noise in radio networks is increased and that this advantage over other, non-adaptive, speech codecs can be capitalized on in several different ways, e.g. by enhancing speech quality or improving spectral efficiency. AMR codecs are standardized by 3GPP for the Global System for Mobile communication (GSM), currently the world's most widespread cellular technology, as well as for Wideband Code Division Multiple Access (WCDMA).
Narrowband AMR consists of eight codec modes with different source bit rates, from 12.2 kbps down to 4.75 kbps. For AMR in GSM, a number of codec modes are collected into a pre-defined Active Codec Set (ACS), which is fixed during a call. The level of channel coding is adjusted while the total bit rate is constant. Consequently, the lower the source bit rate becomes, the more robust the codec is against bit errors. For codec mode adaptation, the receiving side performs link quality measurements of the incoming channel yielding a Quality Indicator (QI), which is defined as an equivalent Carrier-to-Interferer ratio (C/I) [3]. The QI is then compared against a set of fixed, pre-defined thresholds to decide which codec mode to use.